1. Field of the Invention
The invention relates to a Coriolis mass flow meter.
2. The Prior Art
Such a Coriolis mass flow meter is disclosed in EP 1 985 975 A2. This comprises two U-shaped measuring tubes, through which the fluid to be measured flows. A vibrator causes the measuring tubes to vibrate in a defined manner. By means of two vibration sensors the movements of the two measuring tubes at different points is ascertained. The mass flow is calculated from the phase shift of the vibrations measured by the two vibration sensors.
The currently used mass flow meters are also used for low-density fluids, for example gases, or for fluids exhibiting a very high viscosity, the mass flow prevailing with both types of fluid mentioned being usually very low. For this reason, the phase shift between the two vibration sensors is generally very small. Thus, in order to make it possible to measure small mass flows very accurately, the zero point stability of the mass flow meter must be high. To achieve this high zero point stability, it is necessary, inter alia, for the mass flow meters to show minimum vibration in the region of the joint, such that virtually no vibrational energy is dissipated into the adjoining process line. If the forces that are produced by the vibrating measuring tubes are not completely compensated for, the entire housing accommodating the measuring set-up can be caused to vibrate. Thus the vibrational energy can be passed on to the adjoining process line, which in turn leads to feedbacks. Such feedbacks can adulterate the readings. This negative effect may be enhanced when the vibration initiated by the measuring set-up resonates with vibrations in adjacent hydraulic and pneumatic installations. In this case, very strong interaction between the device and the environment may occur, which can in turn lead to considerable measuring errors.
In order to increase the zero point stability in conventional Coriolis mass flow meters, it is known to attach cross braces between the individual measuring tubes. These cross braces then interconnect the U-shaped measuring tubes at their outer tube portions such that the relative position of the measuring tubes is kept constant at least in these outer tube portions. They have the task of separating the natural self-excited vibration of the measuring tubes, such as occurs in the case of non-flowing fluid, from vibration caused by Coriolis forces in the case of flowing fluid, and to reduce the transference of oscillations between the measuring tubes and the piping system. The aforementioned EP 1 985 975 A2 is concerned with the question as to how such cross braces should be precisely arranged in order to damp the vibration as far as possible. The cross braces are attached to each other by conventional connecting methods, such as by adhesion, by welding, or by the use of pin-and-socket connectors. All of these connecting methods are, however, subject to very high manufacturing tolerances. As a result, each joint is to a certain extent a unique entity, which in turn has an individual transmission characteristic in terms of the vibration produced. In this respect, certain vibrational frequencies are readily propagated through the joints of some measuring devices, while in the joints of other measuring devices these vibrational frequencies are damped to a greater extent.
It has been found that in spite of the already proposed solutions regarding the zero point stability there is still room for further improvement.